This invention relates generally to xerographic copying methods and apparatus, and more particularly, it relates to the heat and pressure fixing of particulate thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to provide a combination of heat and pressure sufficient to condition the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing it to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. In order to accomplish fusing in the manner described, the surface temperature of the heated fuser member is maintained at a temperature on the order of 330.degree.-400.degree. F. The specific temperature is usually referred to as the setpoint and where, as is customary, all of the thermal energy is provided from an internal heater, the setpoint is maintained within .+-.5.degree. F. It will be appreciated that from the standpoint of cost and design simplification, a larger setpoint tolerance is desirable. It has been found that such precise setpoint requirements can be relaxed if an external source of heat is supplied for maintaining the surface temperature at the required value during using along with the provision of an internal source of heat for use during standby or warm-up.
In the higher speed copier environment (i.e., 12-20+in./sec) where a relatively thick (i.e. 10 mils or greater) outer insulative layer is utilized for enhanced copy quality and self-stripping, heat and pressure roll fusers can be employed. However, the limitation that all of the heat be supplied from an internal source would preclude satisfactory use of known roll fusers in high speed machines. This is because at the higher speeds with an internal heat source alone the interface temperature between the core and the rubber would exceed the upper temperature limit that the contemplated rubbers could tolerate. The fuser arrangement discussed in the preceding paragraph can be satisfactorily used in the higher speed environments.
An additional advantage is also derived by supplying warm-up requirements solely by an internal heater while providing the thermal requirements for the run mode via an external heater in that the heated roll doesn't have to be rotated during standby warm-up. Contrariwise, if warm-up energy is provided by the external source it would be necessary to continuously rotate the fuser roll or in some other way prevent hot spots which would occur if the heated roll were stationary when the external source is energized.